F430 door unlocks immediately after locking

Sounds like an actuator or door latch internal switch, but I don't know of any easy ways of figuring out which is faulty. Wiring checks are an option (at least you don't have to pull the door cards off, just disconnect the connector near the hinge and do resistance checks.

When you push the fob button can you hear any differences in sound on the left and right doors, with your ear to the locks?

There are two microswitches in each door we are interested in. Both are in the latch assembly. One detects that the actuator has actually moved to the locked position. If it doesn't move, both doors are commanded unlocked. The other microswitch detects if the door is actually latched closed (If you slam the doors closed, does it make a difference or lean on one of the doors?)

For wiring checks, I would disconnect the battery (or remove the fuses for the window and doors... PSF6, PSF3 and PSF7, I believe), then disconnect the door plug (near the hinge). Then check the resistance between pins 2 and 3 (with the door locked and unlocked with the key). Locking the door with the key with power off may make locking the door more difficult.

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Note the unusual pin numbering on the door diagram (pins 1~18 are the small pins).
Also check between pins 14 and 15 with the door latch triggered manually i.e. with the door still open, but using a screwdriver to push the latch closed. Make sure the switch resistance changes between latched and unlatched.

Repeat for the other door if no anomalies found.

There may be other microswitches playing up, but just test those two first.